Referring to FIGS. 8 and 9, there is illustrated a schematic cross-sectional view of a conventional high frequency heating apparatus. On a rear wall of casing 1, cooling fan 2 is mounted. Disposed above and in front of cooling fan 2 is magnetron 3 to which blast duct 5 for guiding cooling air to heating chamber 4 and air guide 6 for guiding cooling air to magnetron 3 are mounted. Disposed under magnetron 3 is inverter power supply 7 on which semiconductor switching device 8 and high voltage transformer 9 are mounted adjacent to cooling fan 2. Cooling guide 10 is provided on another at a side portion of inverter power supply 7. Some of cooling air blown from cooling fan 2 is guided to magnetron 3 by air guide 6 to flow into heating chamber 4 through blast duct 5. In the meanwhile, the remainder of the cooling air is directed toward inverter power supply 7 to flow along a path defined by the surface of inverter power supply 7 on which semiconductor switching device 8 and high voltage transformer 9 are mounted and, cooling guide 10 disposed on the outer side portion of the inverter power supply 7 adjacent to casing 1 and a sidewall of heating chamber 4, so that it cools down semiconductor switching device 8 and high voltage transformer 9 of inverter power supply 7 (see, for example, Japanese Patent Laid-Open Publication No. H2-244587).
Further, as shown in FIG. 10, there has been proposed another conventional high frequency heating apparatus wherein a cooling air produced by cooling fan 103 is axially directed by orifice 104 to cool down magnetron 101 and inverter power supply 102 (see, for example, Japanese Patent Laid-Open Publication No. H8-31562).
Nowadays, in order to satisfy demands for fast cooking, the heat output of the high frequency heating apparatus is increased by an increase in input power, thereby resulting in an increase in heat generated by the semiconductor switching device and a semiconductor rectifying device of the inverter power supply.
However, in such a conventional structure in which the cooling air uniformly flows throughout the inverter power supply, it is difficult to effectively suppress an increased heat generation of the semiconductor switching device and the semiconductor rectifying device of the inverter power supply due to the increased output, which may cause thermal failures thereof, thereby leading to a malfunction of the inverter power supply.
For example, in case semiconductor switching device 8 and high voltage transformer 9 are not enclosed by a top cover, the flow rate of the cooling air flowing along sidewalls of an airflow path tends to be greater than that of the cooling air along semiconductor switching device 8 and high voltage transformer 9 and some of the cooling airflows above semiconductor switching device 8 and high voltage transformer 9. Accordingly, semiconductor switching device 8 and high voltage transformer 9 cannot be cooled down efficiently.
Further, as the output of inverter power supply 7 becomes higher in order to speed up cooking, the heat generation due to switching loss of semiconductor switching device 8 is increased. The increased heat generation, however, cannot be sufficiently cooled down in such a conventional structure, and therefore, the thermal failure of the semiconductor switching device 8 is likely to occur rather frequently. Accordingly, a time period during which the high output is produced becomes short, thus failing to achieve the fast cooking.
Alternatively, because of the heat generation due to the switching loss of semiconductor switching device 8, cooling fins for radiating heat therefrom are required to be larger. Further, since temperature of winding of high voltage transformer 9 becomes higher, the diameter of the winding is required to be greater, which makes high voltage transformer 9 bulky.
Accordingly, in such a case, inverter power supply 7 and hence the high frequency heating apparatus becomes undesirably larger. In addition, since a blowing capacity of cooling fan 2 should be increased, wind noise becomes also undesirably increased.